It is known that polyvinyl alcohol, which is a typical water-soluble polymer, forms a hydrogel. In order to utilize the easy-handling and other characteristics of hydrogels of this type, various hydrogels of polyvinyl alcohol and processes for the preparation thereof have been proposed. However, applications of hydrogels of polyvinyl alcohol are limited because the mechanical strengths are generally insufficient. To solve this problem, it has been proposed to introduce a cross linked structure into hydrogels of polyvinyl alcohol. There is a great risk of losing the desirable characteristics of the polyvinyl alcohol hydrogel as a result of introducing cross linked structure. Also the cross linking process per se is complicated.
Under these circumstances, a process has been developed in which polyvinyl alcohol is subjected to a low-temperature treatment to promote crystallization and cause gelation. According to this technique, a pure polyvinyl alcohol hydrogel having improved mechanical strength can be obtained.
It is known that a polyvinyl alcohol hydrogel absorbs water and that the hydrogel has such properties that it expands and its volume increases when it absorbs water while it contracts upon loss of absorbed water. At present, the potential of utilizing such properties as changing volume and good compatibility with human bodies, to develop products that can be applied to various uses is being studied. For example, an artificial muscle utilizing the force accompanying the volume change and a micro-pump for gradually releasing medicine activated by contracting the hydrogel as water is lost are being studied.